Purification, pH-Dependent Conformational Change, Aggregation, and Secretory Granule Membrane Binding Property of Secretogranin II (Chromogranin C)

Abstract

Secretogranin II (SgII) is one of the three major proteins, the other two being chromogranins A (CGA) and B (CGB), of secretory granules of neuroendocrine cells. The Ca(2+) storage proteins CGA and CGB not only are coupled to the IP(3) receptor (IP(3)R)/Ca(2+) channels that exist on the secretory granule membrane but also are known to play key roles in secretory granule biogenesis. Unlike the better studied CGA and CGB, secretogranin II has never been completely purified in the native state and studied. We have therefore purified SgII in native form from bovine adrenal medulla and subjected it to biochemical characterization. Secretogranin II consisted of largely beta-sheet and random coil structures with a low level of alpha-helicity. Like CGA and CGB, it also underwent pH-dependent conformational changes, showing 9.5% alpha-helicity at pH 7.5 and 17.0% alpha-helicity at pH 5.5. Secretogranin II also underwent acidic pH- and Ca(2+)-dependent aggregation, and it was approximately 8-fold more sensitive than CGA to Ca(2+) in its pH-dependent aggregation but was 8-fold less sensitive than CGB. Further, similar to CGA and CGB that had interacted with the secretory granule membrane at the intragranular pH 5.5, SgII also interacted with the secretory granule membrane at pH 5.5 and dissociated from it at near-physiological pH 7.5, implying similar roles of SgII in the cell as those of CGA and CGB. Secretogranin II hence appeared to actively participate in secretory granule biogenesis as has been proposed for CGA and CGB.

Sorting of the Neuroendocrine Secretory Protein Secretogranin II into the Regulated Secretory Pathway

“…One possibility is that membrane-anchored SgII might compete with a multimerization/ aggregation step common to both models for sorting, behaving as a bait for the nucleation and growth of an aggregate of preysoluble SgII, and recycle such aggregate within the trans-Golgi/ TGN, thereby impairing further trafficking into the regulated secretory pathway. In the simplest scenario, membrane-anchored SgII would homomerize with endogenous and/or exogenous soluble SgII (GFP-or EAP-labeled), which is consistent with studies in vitro reporting pH-and Ca 2ϩ -regulated selfaggregation of SgII (24,44). However, the situation could be more complex, with a role of heteromeric association with other cargo proteins to the overall multimerization/aggregation process.…”

Evidence for the Existence of Secretory Granule (Dense-Core Vesicle)-Based Inositol 1,4,5-Trisphosphate-Dependent Ca2+ Signaling System in Astrocytes

“…The polyclonal anti-rabbit chromogranin A (CGA), chromogranin B (CGB), secretogranin II (SgII) antibodies were raised against purified intact bovine CGA, CGB and SgII [49], [50], and affinity purified against bovine CGA, recombinant CGB and SgII [51]. The specificity of the antibodies was confirmed [50], [52]–[54].…”

Chromogranins A and B as Regulators of Vesicle Cargo and Exocytosis

“…(Yoo 1996;Yoo and Albanesi 1991). However, a similar interaction with soluble species such as catecholamines and ATP is likely to occur as the presence of multiple dibasic groups in their structure increases their ability to concentrate solutes (Yoo and Albanesi 1990;Yoo 1996;Park et al 2002).…”